Sensors for angular measurement of a rotating member, such as an automobile tire, camshaft, crankshaft, steering wheel, etc. are common. Magnetic field sensors are often preferred over other sensor types due to their robustness and low production costs. A magnetic sensor typically includes a rotatable element and a magnetic field sensor that is stationary relative to the rotatable element. The rotatable element defines teeth or is magnetically coded around its edge, and as the toothed or magnetically patterned regions pass the sensor a magnetic field is induced. The normal component of the induced field at the position of the sensor has a sinusoidal-like shape.
The magnetic field sensor element (for example, Hall-effect sensor, Giant Magneto Resistance (GMR) sensor, etc.) converts the applied magnetic field into a proportional electrical signal. Signal processing, such as zero-crossing detection, is used to convert the sinusoidal-like signal into a binary sequence that is a digital representation of the pattern on the wheel. Knowing the pattern, the rotational speed and angular position can be determined from this binary signal.
Various factors such as packaging and mounting tolerance, mechanical vibrations, temperature variations, defects in the teeth or magnetic pattern, etc. can cause variations of the electrical signal shape, such as displacement of the peak and zero value positions in the signal. In turn, these factors can cause measurement errors.
For example, automotive antilock brake systems (ABS) measure the speed of rotating tires using magnetic sensors. If the ABS sensor detects a change of the speed of a tire, it takes corrective action. If the magnetic sensor system provides an incorrect speed indication, the ABS could activate unnecessarily—even if the wheel speed is correct.
For these and other reasons, there is a need for the present invention.